Apparatus for modifying the transfer characteristics of a vacuum tube



Jan. 22, 1952 o, H. SCHADE APPARATUS FOR MODIFYING THE TRANSFERCHARACTERISTICS CF A VACUUM TUBE 2 SHEETS-SHEET l Filed DeG. 9, 1947 E mWw A an MT ./N mw w UC .o0 g mw mw RFE@ 5 5 C4 I Mul@ ad C lrx C m m? @mP Jr" m Cz/U G 5.2K q. 5 m 6 I 3 D T590 I me rw, /I

Jan. 22, 1952 o. H. SCHADE 2,583,345

APPARATUS FOR MODTFYTNG THE TRANSFER CHARACTERISTICS OF A VACUUM TUBEFiled DBC. 9, 1947 2 SHEETS-SHEET 2 W6/1ML OUTPUT Patented Jan. 22, 1952APPARATUS' FOR MODIFYING THE TRANS- FER CHARACTERISTICS OF'A VACUUM TUBEOtto H. Schade, West Caldwell, N. J., assignor to Radio Corporation ofAmerica, a corporation of Delaware Application December 9, 1947, SerialNo. 790,582

i 1 The present invention relates in general to amplifier tubes and,more particularly, relates to means for modifying the-.transfercharacter- ...istics of such a tube by the use of a non-linear cathodeload resistance. vItis frequently desirable in electronic circuits toemploy an amplifier tube the conduction characteristics of which aresuch that' the output of the tube will be generally proportional to theinput over a portion of its operating range only. -Such a characteristicis particularly useful in .circuits designed to limit the amplitude ofan input signal to acertain maximum value. In radio receiving systems,rfor example, a circuit of this type aids inthe elimination of noise orother spurious signals which extend above the t amplitude of the signalconveying the desired intelligence.' r

Another adaptation of a circuit of the above nature is in the televisioneld where itis desired tocontrol the contrast of the reproduced image bymeans known as gamma correction. In order to act as a satisfactory gammacontrol, the circuit should Vprovide maximum non-linearity in thatportion of the tube operating range giving maximum gain. Thishaspreviously been ac-` A still further advantage obtained by modifying thecharacteristic curve of an amplier stage ""so thatthe-amplifier tubewill provide different '--degrees of `ampliiication in differentportions lof its operating range Iis that the portion of a telef visio'video 4'signal which contalnsthe synchrolnring information and blacklevel may be eX- -panded in' amplitude as contrasted with that portionof the television signal containing the 'image intelligence or videoinformation. This is important to counteract compression of this portionwhich may occur in the video amplier or transmitter modulator.

As above mentioned, attempts have previously `been made to accomplishsimilar results lby uti- ."lizing one or more electron discharge devicesin rthe output circuit of the amplifier tube and by so arranging theseelectron discharge devices .that they will beconductive within variouspredetermined portions of the total operating range fof the ampliiier.Such. an arrangelfn'crlt has 8 Claims. (Cl. 179-171) serious drawbacks,however,

due to the fact that an adequate output from the amplifier tube requireseither 'a Vtube oflarge rcapacity or Aa decreased load resistor, orboth. TheA use of a large tube is undesirable-since'it increases thecostof the apparatus, and the use of a small load resistor or impedancecuts down the signal output. These expediente are needed to counteractthe loss of high frequencies because. of the various added shuntingcapacities in these circuits.

It has been found that the. above disadvantages may beeliminatedwithout` the employment of additional components by utilizing avariableimpedance electrondischarge device .connected in the cathodecircuit of the ampliertube in parallel with the tube cathode resistor.Furthermore, measurements have shown that the gain and signal outputcapability ofthe system Tis increased to,a considerable kdegree bytheuse of such an arrangement." This results from areduction in theshunting capacity in the 'output circuit of the Vamplier. tube which, inturn, permits the employment ofha higher value of load resistance. Stillfurther, the output power requirements of the. amplifier tube Amay incomparison withother circuits in many cases be reduced with a consequentsaving in cost.

One obiectof the present'invention, therefore,

1 is to provideimproved means for modifying the transfer characteristicsof an amplier tube.

AnotherA object of the invention is to provide an ampliiier tube with anon-linear vcathode load resistance.

A further object of the linvention is to provide an amplifier tubej witha cathode load impedance consisting of thevparallel `combination of aresistor and a diode, the latterbeing so biased Aasto cut olv suddenlyor gradually when the current in the amplifier tube reaches apredetermined level."

A still further object of the invention is to provide an amplifier' tubewith a cathode load impedance comprising a resistor in parallel with agrid-controlled electron discharge device, and to control the conductionof this electron discharge device by placing a suitable bias voltage onthe grid thereof.

An 'additional object of the invention isto provide an amplifier tubewith a cathode load impedance consisting of a resistor in parallel witha plurality of electron discharge devices which are s'o biased as to'cutoff 'at -dile'rent values of current ilow through the amplifier of theinvention and from the drawings, in which:

Fig. 1 is a circuit diagram of a preferred form of the presentinvention;

Figs. 2 and 3 are graphs explaining the operation of thecircuit ofFig.1; Y

Fig. 4 isa circuit diagram of a modification of Fig. 1;

Figs. 5, 6 and '7 are graphs explaining the opration of the circuit ofFig.Y 4; l

Fig. 8 is a circuit diagram of afurth'erfmodification of Fig. 1;

Fig. 9 is a family of curves useful in explaining the operation of thecircuit-fof Fig. 8;

Fig. 10 is a circuit diagram-of a further modiiication of Fig. 1; and

Figs. 11 and 12 are graphs. illustratingcertain characteristics of thecircuit of Fig. 10. y

Referring rst to Fig..1 there is shown an arnpliiler tube V havingaplate p, a control grid g,

and a Vcathode k. The control Agrid 'g-of tube V isadapted to receive'aninput signal, `while the plate "p is connected to some suitable:utilization circuit (not shown). A diodeD has its-cathode connected Vasshown tothe vcathode 7c' of tube V, andits anode connected vto a pointof positive polarity on 'a ybattery or other source of positive biasvoltage En. The negative'terminal of battery YEn is grounded. A'resistorRis connected between the cathode kof-tube V and ground so as to beeffectively in's'huntwith thel series cornbination oi .diodeD andVbattery En. A further resistor Rg 'is connected lbetween'the grid g ofVtube V and some point on the resistor R. l It will now be seenthattthesignal voltage Eg V`required to produce a given plate current in thecurrent 'through diode D 'is `atl-a maximum when the l.current through.tubeVfis zero. In

other Words, vthe .diode current .is .greatest vvhenf.l

the amplifier .tube iscut off by an` applied .signal Eg. Conversely, thecurrent through diodeV D becomes zero .when .the .voltage at point'Mybecomes positive .by .an `amount equal to .the bias voltage En asaresult 'ofahigh value. ofcurrent.. Vilowhthrough the amplifier tube V.

Such operation` can .bernore clearly understood by .reierencetovthe'curvesgof Fig.. 2, in lwhich the normal plate or cathode vcurrentof `tube VV (neglecting the. diode D) .isdesignatedby Ithe sym. .bol Ik.The point v.ofintersectionoi thecurvelk with the voltageaxis isrVdetermined by Athe Amagnitude of the bias voltage Ee.. applied Ato thelgrid g. As aresultvof the presence ofthe diodev D,

however, the @resultant characteristic of the-cath-.-

ode or plate current of tube'Vas plotted against the signa-l linput-Egwill b e as shown by thecurve Ik. This curve Ifk Vrepresents the -sumfoi the currents through the 'two l.tubes V and D Vplus lthe currentthrough the resistor vR. Currents will 'flow through all of ytheseelementsuntil such `time as the-current flow through tube V .(Yas theresult of an increased signal input Eg) reaches a point where thevoltage at M zcutsfo" the diode current. This 'cut-off :pointfisrepresented 4 by the bend, or knee, in the characteristic curve Ik. Thelocation of this bend, or knee, in the tube characteristic is obviouslya function of the Value of diode bias En which, in turn, determines thevalue of current In at which the diode cuts *.off. lBeyond this ,bend4in 'the characteristic curve, the slope is linear, since the totalcurrent of V then iows through the resistor R alone.

Fig. 2 additionally shows the characteristic curve obtained for currentand voltage in the cathodecircuit of`V. This curve is designated as I"1As in the `case of curve Ik, the slope is substantially that of thediode characteristic to lthe current cut-ofi.' level in the diode whichis,

as previously stated, determined by the bias voltage En. Beyond the kneeonly the value of resistorv R' is a -factor in determining the slope ofare drawn for Various diode current cut-oi! conditions Inl, In?, and In3obtained by progressively increasing the positive diode bias Voltage En.For the condition In3 adecrease in value'of resistor R causes acorresponding change in the slope of the tube characteristic as shownyby the broken line 3b. A still further decrease in the value ofresistor`R gives the broken line characteristic portion 3c. An increasein resistor value, however, changes the'slope in the manner shown by thebroken line 3d.

In Fig. 4 there is shown a modification of the circuit of Fig. 1 inwhichthe connections to the diode D are reversed, so that the anode of thediode isconnected to the cathode 1c of tube V and the cathode of thediode connected to the positive terminal ofthe source oi bias voltageEn. The result of such a reversal in polarity of the diode is showngraphically in Fig. 5. From the latter iigure,'it will be seen that,instead of conducting during the initial portion of the signal appliedto the gridg of tube V, the diode D remains cut 01T until such time `asthe iiow of current through the amplifier tube increases the positivevoltage at pointMto a valueabove that of the negative bias'battery En.In other words, the current characteristic I"k is linear and coincideswith the resistance line R between the voltage limits of zero and En Atthe latter limit, however, the diode D begins to conduct, and thecathode current of tube V is divided between the resistor R and thediode D. As a consequence, the curve I"k bends at the voltage limit Enand follows a path such as V:indicated in Fig. 5. As inthe case of Fig.2, the slope of the resistance line. R .is determined by the particularvalue ofresistance chosen.

Fig. .6 illustrates .the .effect of varyingthe value of the bias voltageEn inFig. 4. .By-increasing the value of thisfbias potential, lthe kneeor bend in the characteristic curve-of the amplifier tube may be made tooccur further along the Voltage axis.

Fig. 7 shows the manner in whichthe initial slope of the tubecharacteristic of Fig. 6 is dependent uponA the particular value ofresistance chosen for R. By decreasing the value of this resistance, theangle made by the initial portion ofthe tube characteristic lwith thevoltage axis is progressively'decreased.

Fig. 8 illustrates a further modication of Fig. 1 utilizing a triode V2in shunt with the resistor R. rather than the dodeD'ofFigil. By the useof such a triode, it is possible to vary the cut-off point of the tubeby means of a grid bias battery Ec rather than by varying the positivevoltage applied to the anode of diode D as in Fig. 1.

Fig. 9 illustrates a family of curves explaining the operation of thecircuit of Fig. 8. As in the curves of Fig. 2, the location of the bendor knee is determined by the point at which the triode V2 begins toconduct, and the slope of the current curve subsequent to that point isdependent upon the value of resistor R. The solid line Rin Fig.' 9

shows the linear characteristic ofthe amplifier' forth. A pair ofelectron discharge ,devices D1v and D2 are connected functionally inparallel' relation. As shown in the drawing, the electron dischargedevice D1 is connected to an adjustable source of bias potential Enl, sothat the current flow Ik in the amplifier tube V will have an initialbend or knee at the level Inl at which the diode D1 begins to conduct.This level In1 in turn vis dependent upon the value of bias voltage Epl.

A second. electron discharge device has its anode connected to the biassource Epl through an adjustable resistor which eiectively acts as asecond source of bias potential E112. By suitably adjusting thisresistor to vary the bias voltage E192, the diode D2 may operate to cutoff at a different point than the diode D1, and thusl produce a secondknee or bend in the current curve Ik of tube V at the level 1192. Asshown in Fig.,11, this current level In2 is determined by the value ofbias voltage En2 similar to the manner in which the bias voltage Epldetermines the currentlevel Inl. It will thus be appreciated that Fig.10 provides means whereby the transfer characteristicsI of tube V may bechosen so as to provide selective control over the output of theamplifier tube as a function of the amplitude of the input signal. Itwill be obvious that additional electron discharge tubes in parallelwith D1 and D2 may be added so as to bring about additional points of asa direct function of an increase in signall frequency. In thearrangement of Fig. 10 the frequency response is made uniform by the useof an inductor L connected between the 'c'athode 7c of tube V and oneend of resistor B. *,.When omitting or decreasing this inductor L,the.cath ode'impfedance of tube V will decreasefalsv a direct function bffrequency because of shunt fcapacities C in the circuit, thus providinghigher gain at the higherfrequency levels. The circuit components ofFig. l0 may be chosen, for example,v to have the values set forth, inwhich case the circuit will provide maximum gain near the maximumfrequency passed by the circuit, as, forfeiiample, a frequency ofmegacycles (see Fig. 12). 'Ifhis is indirect contrast to circuitspreviouslyknown in theart, in which the shunting. capacity in the outputcircuit causes a falling off of gain at the hi'gher'frequencies ratherthan the,` increase in gain which results from the use of applicantsdisclosure.

It will thus be seen that applicant has provided a system which isextremely useful in cases where the output of an amplifier tube is to becompressed to certain prescribed values relative to input signalamplitude and in which the degree of compression can be made a functionof frequency. Furthermore, the use of circuits such as shown hereinprovides a simple way of bringing about control over the gamma of atelevision signal to'thereby effect proper contrast in thev reproducedimage. Still further, the use of these ycircuits permits remote volumecontrol, or gain control, in circuits utilizing input signals having alow percentage of distortion. It should still further be noted that, bythe use of applicants circuits, it is possible to achieve these resultswithout interfering with the circuit constants of any lter combinationswhich may be utilized in the output of the amplier tube.

Having thus described my invention, I claim 1. In a signal channel, anelectrical signal translating circuit comprising in combination: anelectronic discharge tube having at least an anode, a cathode, andacontrol electrode, a cathode impedance connected between the cathode ofl said electron discharge device and a point of fixed potential, a tapon said cathode impedance, a resistance connected from said tap to saidcontrol electrode, a plurality of unilateral current conducting deviceseach having at least a cathode and an anode, connections from each ofsaid unilateral current conducting devices cathodes to said electrondischarge tube cathode, a plurality of bias supply terminals eachsupplying a difierent value of positive bias voltage referenced withrespect to the point of iixed potential, a separate circuit from each ofsaid unilateral conducting devices anode to a corresponding andrespective one of said plurality of bias supply terminals.

2. In a signal channel, an electrical signal translating circuitcomprising in combination: an electronl discharge tube having at leastan anode, a cathode, and a control electrode, a point of fixedpotential, a negative bias supply terminal referenced to said point ofxed potential, an input impedance connected between said negative biassupply terminal and said discharge tube control electrode, a cathodeimpedance connected between the cathode of said electron dischargedevice and said point of fixed potential, a galvanic connection betweena point on said cathode impedance and a point on said input impedance, aplurality of unilateral current conducting devices each having at leasta cathode and an anode, connections from each of said unilateral currentconducting devices cathodes to said electron discharge tube cathode, aplurality of bias supply terminals each supplying a different value ofpositive bias voltage referenced with respect to the point of fixed'potential, a separate circuit from each of said unilateral conductingdevices anode to a corresponding and'respective one of said plurality ofbias supply terminals.

3. Apparatus according to claim 2 wherein said cathode impedanceincludes an inductive portion and a, resistive portion and wherein apart of said cathode impedance inductive portion is included betweensaid discharge tube cathode and the point on said cathode impedance towhich said galvanic connection is made.

4. In a signal channel, an electrical signal translating circuitcomprising in combination: an electron discharge tube having at least ananode, a cathode, and a control electrode, a point of ilxed potential, anegative bias supply terminal t101 electrode. -acathodeimpedance-connected between-the; cathodeA of 'saidelectronfidischarge. j

devicek and lsaid point ot xed potential-,f agognpedance-and av point-von saidrinputimpedance;

plurality .0f .unilateral current. Conducting devices f each havingat-least agcathode. and anlandde,tl

connectionsffrom each of; said junilateralgcuirent conducting devicestcathodes to said electron disf Y Chareatube Cathode.a;-plurality,Qf-f:tias` Supply-1;

terminaisfeach s uppyllgJv a. different value,off;.- positive?bias`voltage referer-,icedwith respect .1;0,v

the point ofnxed potential,v a A.separate circu from each ofnsaid`unilateraliconducting--devices anodeto .a correspondingq andrespectivefoneof said plurality of biassupplyjterminalsafr l i 5.Apparatus accordingto claim -,Z-Whei'ein said;A

a second capacitorfis -connected from a'pointlon y Vdeyicesto acorresponding `and respective oneoi.;VVT

said feedback resistance -and said pointof-ixed potential;

6. In a signalchanneL an electrical signal@ .l translating circuitcomprising in combination: .l Y an electron discharge tube havingatdeastfananode; acathode, andra controlfelectrodeya cath-L translating circuitcomprising in combination an electron discharge tube having'latvgleastan-I anode,y a cathode, anda control electrode, a point @y u of xedpotential, a negative bias supply terminali.- t o referenced to saidpoint yofy xed potential, aningf u put impedance connectedbetweensaidnegative;v

bias supply andV said dischargetube. controlelec trode, a cathodeimpedance` connected-between, the cathode ofsaid'electrondischarge,leviceiand` v.said point of.xed'potentialffa-galvanicconnection between a point onsaidlcathodeimpedance and afpointon said-input impedance,l a pluraley v' Vityof unilateral currentconductingdeviceseach f having at least a-cathodeand an 4anode operating `electrode, connectionsfrom an electrode :oneach ent value lof ybias voltage referenced'with respect to they-pointof fixed potential, a; separate. circuit and respecta@oneptsaigburaiityman has 1 a -supplyfterminala 7- Ina -sisnalv1 ialrielman electrical.,.sienat of said unilateral current conducting-fdevicesto. saidelectron discharge tube cathode, a plurality :f of bias supplyterminalsfeach supplyingfafdifferfor each of said unilateralvcurrentconduction,P

said plurality. of said bias supply terminals'. 8. Apparatus accordingto claim -7 wherein said t,

cathode impedance includes aninductiveportiont ,Y

and a resistive portion and wherein a partofsaid ode impedance connectedbettveenthe cathode of said electrondischarge:device and apoint ofxedpotential, atapon sai-d cathodei-Inpedance, .y a resistance connectedfrom said tap to said conf trol electrode, apIurality oi.unilateratcurrent 35 conducting devices each having atleast a catn-` lode and an anodeloperatingeleotrodeconnecs l tions from one` electrodefon each of said unilateral currentconducting devices to said. elecl Y-tron discharge tube cathode! arplurality, ofhbiavs supply,A terminalseach supplying a diierent valueof lbias voltagereferenced withrespectrto the pointof xed potential, .a vseparate circuit, o.

cathode impedance inductiveportion lis.included`.f fl between rsaiddischarge .tube cathodaand the point-'on said cathode impedance, towhichsaid 4galvanic connection is made...

OTTOYH; SCHADE. l t. o

REFERENCES CITED ,f

The following references'are onrecord'intheffile of thispatent:

UNITED STATES plyrglgyrs4 a .Y

